Thin film materials made of thin sheets of atoms are novel structures characterized by unique specifications that can be used to improve electrical, mechanical, thermal, and optical properties of other materials. Graphene is an example of such atomically thin materials with many commercial applications, either proposed and/or under development in a variety of technical areas. The behavior of transmitted and reflected electromagnetic waves when they interact with thin film materials depends on several properties, most importantly, on their sheet conductivity, in such a way that areas of higher sheet conductivity are characterized by higher reflectivity and areas of higher transmittance are related with lower sheet conductivity of this material. The conductivity of the material and, thus, transmission and reflection coefficients are frequency dependent parameters and they can be measured in a wide range of the frequency spectrum, including ultra-violet range, visible range, infrared range, terahertz range, millimeter-waves and microwaves. The sheet conductivity of the thin-film is related to the average transport properties (such as the carrier density and mobility) of the material and can be used to characterize its electrical continuity and uniformity. The small-scale and large-scale measurement of the conductivity is an important issue in the non-contact quality inspection of thin film materials.
Several methods exist to inspect the quality of thin films, in some cases by measuring the conductivity of this material. One typical method used is the micro four-point probe, also known as the Van der Pauw technique. This measurement technique is characterized by its low reliability and spatial resolution as it provides a single value of DC conductivity of the whole sample (see “Graphene Conductance Uniformity Mapping”, Buron et al., Nanoletters, 12 (10), pp 5074-50812012). In addition, this characterization method belongs to the group of invasive characterization methods, as some metallic contacts are required to perform the measurements. Another method used is confocal Raman spectroscopy. It should be noted that confocal Raman Spectroscopy is only used for active Raman materials. This technique provides information about the defects, doping density, mechanical strain, and the number of thin film layers in the sample by performing micro-scale measurements. It is important to remark that confocal Raman spectroscopy is characterized by low throughput as the acquisition time is limited by the low efficiency of Raman scattering and the size of the single point is restricted to the spot size of the laser used (around 500 nm). Thus, in order to characterize the material, the time-consuming raster scan needs to be performed across the entire sample. Consequently, Raman spectroscopy is not an adequate method to characterize large-scale samples (≥10 cm2). A third method is optical imaging. The optical imaging merely provides quality information about the full area of the sample. Moreover, it does not provide any quantitative information about the distribution of the conductivity in the sample. Another method used for the characterization of thin materials is the use of an Atomic Force Microscope (AFM). It can provide a topological map of the surface of the sample but it requires several hours to map a single sample with an area of 1 cm2. Moreover, the usage of this measurement technique may lead in some cases to damages in the material under test. Finally, Transmission Electron Microscopy (TEM) is also used to get the information about the quality of thin film materials. For instance, in order to analyse the graphene, it has to be transferred onto 3 mm TEM grids. This technique provides a lot of information about the graphene material such as grain size, grain boundary structure, number of graphene layers, etc. however it is a destructive technique and only small areas can be investigated
Due to the rapid growth of the thin film material market and the necessity of obtaining thin film materials of larger areas, brand new methods are required in order to provide fast and reliable techniques to characterize the quality of the fabricated materials both inside and out of the production line. Thus, the conductivity of thin materials can be used as one of the parameters to assess the uniformity of the material. Moreover, this system may be used to test the reproducibility and repeatability of the fabrication process for small (<10 cm2) or large-area (≥10 cm2) thin film materials (see “Terahertz Graphene Optics”, Rouhi et al., Nano Res, October 2012, Volume 5, Issue 10, pp 667-678).
Several examples included in the literature present the methods to calculate the conductivity of thin film materials using the transmission configuration, which is often not appropriate in an industrial process as it requires placing the radiation emitter and detector at the opposite sides of the measured material. Thus, a technique that allows a more practical implementation of this type of inspection is very desirable.